1981 desirability
Here are my thoughts on the 1981 model. I have one (red of course) and as a general statement think it is fair to say there are more similarities than differences to the earlier Series 2 versions. I also think they are fairly similar to the early Series 3 in general driveability, except the fuel management systems are more advanced in the Series 3.
Styling of the earlier Series 2 models is nicer in my opinion. The bumper styling design is better and the materals of construction hold up better over time.
For the sake of discussion, since these are all using the same basic 2L 4 cylinder engines with 5 speed transmissions, in my opinion the differences in power and drivability to other model years are negligible. We are not talking muscle cars with significant drivetrain and suspension options here. We are talking about a few horsepower, slight gearing changes, and suspension geometry tweaks over the years in question. You are still going to be rowing thru the gears while out and about (a good thing, to me), and at all legal speeds or slightly above, differences in rpm while cruising in 5th gear seem slight to me.
While out on drives with owners of earlier series 2 and mid-80's Series 3, I have not noticed any big differences in handling or acceleration.
One thing I have noticed on various threads is that the factory was big on using up all inventory prior to switching over to the Series 3, so there are some differences in 1981 models in such areas as consoles (mine is like the 1982, not like the 1980 and many 81's. Also, mine has factory AC, even though much of the literature says it wasn't introduced until 1982. These types of differences are merely one of preference, because there are parts available regardless. For example, my original seats needed reupholstering, and I changed them out for 1985 seats in better condition, they fit right in with no alterations necessary.
Things to look for regarding mods:
Possibly converted to dual carbs, usually Webers, from the SPICA fuel injection. Lots of threads here on the pros and cons. Mine was converted by 2nd owner, I am the 4th. I like the sound better, but get lower gas mileage, have had no issues in over 8 years, just leave them alone (I don't drive in winter, when fuel injection is a big plus).
May have been desmogged, air pump, etc. removed. Mine started out as a CA car, but was desmogged before I got it. A 1981 is over 25 years old, so I believe in most states emissions testing will not be an issue, others can comment? Ditto for the presence of the original catalytic converter setup.
Same rust issues as all others, need to check out the rocker areas in particular, jacking points, etc.
Bottom line, if it is in decent condition I think there is no reason not to consider a purchase.
These are the first things that come to mind, hopefully others will weigh in with their thoughts.
John